A technology for preventing damage or abrasion on surfaces of a metallic material is classified roughly into the surface transformation method and the surface coating method.
The former technology includes a method of forming an alloy layer on surfaces, such as the cementation method, the nitriding method, the metallic cementation and the like, or quench hardening by heating, a method making use of residual stress, or the shot peening method. These methods involve complicated and large apparatuses therefor, and are inapplicable or difficult to be applicable to a base material for a shaft, which is composed of a soft metal such as aluminum or the like.
In contrast, the latter technology includes a metal plating method typified by hard chromium plating, the metalizing method, the ceramic coating method, which uses ceramics such as cermet, but involves a disadvantage in poor durability due to a difficult point in its bonding strength for a base material for the shaft, which is composed of a soft material such as aluminum or the like, as well as relatively low productivity.
Also, as a surface treatment method for an aluminum alloy, a method is known which comprises applying the electrochemical treatment (anodic oxidation method) on surfaces of the alloy to form a protective film composed of aluminum oxides and having a thickness of several to several tens microns to enhance its surface hardness. However, such protective film is small in thickness to be liable to wear, and has less chemical resistance to be liable particularly to erosion by alkali. In addition, there is caused a problem in cost because it takes considerable time to form such protective film.
Further, the above-mentioned various methods apply to metals as the base material, and are not very applicable to synthetic resin materials.